Many modern day aircraft employ gas turbine engines. Accordingly, it is well known that gas turbine engines include a fan, a compressor, a combustor and a turbine. The serial flow combination of the compressor, the combustor and the turbine is commonly referred to as a core engine. Once air enters the core engine it is pressurized in the compressor. The pressurized air is then mixed with fuel in the combustor. This mixture is subsequently burned, which generates hot combustion gases that flow downstream to the turbine. In turn, the turbine extracts energy from the hot combustion gases to drive the compressor and fan. The excess hot combustion gases, not used by the turbine to drive the compressor and fan, are discharged from the core engine through an annular exhaust nozzle, which produces thrust that contributes in powering an associated aircraft. In addition to this thrust, a much larger amount of thrust is generated by the fan taking in ambient air, accelerating that air and discharging it from a fan exhaust nozzle. This thrust from the fan exhaust nozzle provides the majority of propulsion thrust for the aircraft.
In an effort to increase efficiency, gas turbine engines have evolved to produce greater thrust while, at the same time, consuming less fuel. For the most part, achieving greater thrust on less fuel consumption is based on, among other factors, controlling the speed and direction of the fan generated thrust and the core engine generated thrust. The cross sectional flow areas of the core engine and fan exhaust nozzles generally determine the speed of such flows. It follows that regulating the cross sectional flow areas, by either pre-selecting the size of fixed area nozzles for theoretical engine operating conditions or utilizing variable area exhaust nozzles which can be adjusted in area for ideal flow throughout a range of operating conditions, will achieve control of flow speed. As for directional flow control, it is generally controlled by the specific geometric shape of the nozzles. Further details can be found in commonly assigned U.S. Pat. No. 7,900,433.
Typically, a gas turbine engine has a nacelle, which includes a core engine cowl and an outer fan cowl. The core engine cowl provides an aerodynamically contoured cover for the core engine. This core engine cowl extends around the core engine and terminates at the downstream end thereof at the engine exhaust nozzle. The outer fan cowl surrounds the core engine cowl and the fan blades. In this configuration, a fan duct, which terminates downstream at the fan exhaust nozzle, is functionally defined by the area between the outer fan cowl and the core engine cowl.
With most commercial transport aircraft today, the engine and nacelle are attached to the underside of the wing by a structural pylon that is surrounded by an aerodynamic fairing. Commonly, the pylon is positioned in such a way that it cuts across the top of the fan nozzle. This orientation typically causes a partial area blockage in the fan nozzle. As can be appreciated from the discussion above, this obstruction to the cross sectional flow area of the fan exhaust nozzle has a negative impact on efficiency. Additionally, the pylon obstruction creates circumferential flow migration with associated aerodynamic loss and non-axial thrust vector.
Thus, there is a need for a fan nozzle that accommodates for this pylon blockage in order to improve thrust efficiency and thrust vector.